[Halld-cal] [EXTERNAL] FCAL calibration follow-up

[Shepherd, Matthew]

Igal,

The procedure breaks down about 4 GeV because one can't easily analyze a symmetric 8 GeV pi0 decay with 4 GeV clusters -- they merge.

There are two potential routes to higher energy that have been discussed:  fix energy of one photon in pi0 decay to say 1 GeV and use second to probe energy dependence allowing reach to potentially higher energy.  Or use something else like Compton.  Both of these good ideas have been put forward by the PrimEx team as solutions, but to my knowledge, no one has tried them yet.

Matt


On May 1, 2020, at 10:25 AM, Igal Jaegle <ijaegle at jlab.org<mailto:ijaegle at jlab.org>> wrote:

Matt,

Slide 4 shows that the current energy dependence parametrization is wrong above 4 GeV or this behavior is due to overlapping clusters? If it is the latest, how can you be sure that the current parametrization is correct?

tks ig.
________________________________
From: Shepherd, Matthew <mashephe at indiana.edu<mailto:mashephe at indiana.edu>>
Sent: Friday, May 1, 2020 10:07 AM
To: Igal Jaegle <ijaegle at jlab.org<mailto:ijaegle at jlab.org>>
Cc: Hall-D Calorimetry <halld-cal at jlab.org<mailto:halld-cal at jlab.org>>
Subject: Re: [Halld-cal] [EXTERNAL] FCAL calibration follow-up


Igal,

 Because the pi0 fitting (which is of poor quality) is systematically biased by 1 to 2 MeV when he doing the energy dependence correction the pi0 mass is shifted to lower mass and falls within 2-3% around the pi0 PDG mass and stops there.

Please reference:

https://halldweb.jlab.org/DocDB/0043/004387/001/presentation.pdf

The left plot of slide 4 shows that if the average energy for photons used in making the pi0 peak varies, then the pi0 peak will vary by about +/- 1.5%.  The pi0 mass on the left of slide 2 clearly averages over energies in a way that is location dependent.

It happens even with perfect fitting and in the absence of systematic bias.  It is a pure byproduct of energy non-linearity and variation in average energy of photons from pi0 decays.

Matt

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